This is a first - door lock issue 95 E300, 312k miles
 

When you lock the drivers side front door with the key the inside knob goes down but it does not really lock. All you got to do is pull on the door handle and the door opens. I guess it is time to pull the panel and see if I can figure out what is going on. At least it looks locked:).
Anyone ever seen this?

Did these vintage model MBs have alarms on them, and if so, does turning the mechanism then arm the alarm system, if the car has one?

It does and I just recently plugged it back in. When I had it unplugged it actually prevented the car from starting when the car was locked from the passenger side and then unlocked from drivers side. The wife sets it off every now and then. She just realized today that she was not really locking the door when she locked it with the key on the drivers door.